Fetch datas from another repository

Tachikoma is great framework but I want to manage YAML files separately from tachikoma main repository.
So I added a feature that clone data directory from other location and use it as a `@data_path`.

It works good on my local environment, and it does not affect default behaviour (for the present).

With public data repo: https://github.com/kyanny/tachikoma-data

```
$ export BUILD_FOR=gfm-rendering-compatible-api
$ export TOKEN_GFM_RENDERING_COMPATIBLE_API=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
$ export LOCAL_DATA_PATH=local_data
$ export LOCAL_DATA_REMOTE_URL=https://github.com/kyanny/tachikoma-data.git
$ bundle exec rake tachikoma:fetch_data tachikoma:load tachikoma:fetch tachikoma:bundle tachikoma:pull_request
```

With private data repo: https://github.com/kyanny/tachikoma-data-private

```
$ export BUILD_FOR=gfm-rendering-compatible-api
$ export TOKEN_GFM_RENDERING_COMPATIBLE_API=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
$ export LOCAL_DATA_PATH=local_data
$ export LOCAL_DATA_REMOTE_URL=git@github.com:kyanny/tachikoma-data-private.git
$ bundle exec rake tachikoma:fetch_data tachikoma:load tachikoma:fetch tachikoma:bundle tachikoma:pull_request
```